1. Field of the Invention
The present invention relates broadly to the art of rendering and maintaining a surface of a substrate highly hydrophilic. More particularly, the present invention relates to the antifogging art wherein the surface of a transparent substrate such as a mirror, lens and sheet glass is made highly hydrophilic to thereby prevent fogging of the substrate or formation of water droplets. This invention is also concerned with the art wherein the surface of a building, windowpane, machinery or article is rendered highly hydrophilic in order to prevent fouling of, to permit self-cleaning of or to facilitate cleaning of the surface. This invention also relates to a hydrophilifiable member having a surface layer which is capable of having an extremely small contact angle with water, a method for rendering the member hydrophilic, a method for forming a hydrophilifiable surface layer, and a coating composition for forming a hydrophilifiable surface layer.
2. Description of the Prior Art
It is often experienced that, in the cold seasons, windshields and window-glasses of automobiles and other vehicles, windowpanes of buildings, lenses of eyeglasses, and cover glasses of various instruments are fogged by moisture condensate. Similarly, in a bathroom or lavatory, it is often encountered that mirrors and eyeglass lenses are fogged by steam.
Fogging of the surface of an article results from the fact that, when the surface is held at a temperature lower than the dew point of the ambient atmosphere, condensation of moisture in the ambient air takes place to form moisture condensate at the surface.
If the condensate particles are sufficiently fine and small so that the diameter thereof is on the order of one half of the wavelength of the visible light, the particles cause scattering of light whereby window-glasses and mirrors become apparently opaque thereby giving rise to a loss of visibility.
When condensation of moisture further proceeds so that fine condensate particles are merged together to grow into discrete larger droplets, the refraction of light taking place at the interface between the droplets and the surface and between the droplets and the ambient air causes the surface to be blurred, dimmed, mottled, or clouded. As a result, an image viewed through a transparent article such as sheet glass may become distorted, or the reflected image in a mirror may be disturbed.
Similarly, when windshields and window-glasses of vehicles, windowpanes of buildings, rearview mirrors of vehicles, lenses of eyeglasses, or shields of masks or helmets are subjected to rain or water splash so that discrete water droplets are adhered to the surface, their surface is blurred, dimmed, mottled, or clouded, resulting in the loss of visibility.
The term "antifogging" as used herein and in the appended claims is intended to mean broadly the art of preventing or minimizing occurrence of optical trouble resulting from fogging, growth of condensate droplets or adherent water droplets mentioned above.
The antifogging art can significantly affect safety and efficiency in a variety of settings. For example, the safety of vehicles and traffic can be undermined if the windshields, window-glasses or rearview mirrors of vehicles are fogged or blurred. Fogging of endoscopic lenses and dental mouth mirrors may hinder proper and accurate diagnosis, operation and treatment. If cover glasses of measuring instruments are fogged, a reading of data will become difficult.
The windshields of automobiles and other vehicles are normally provided with windshield wipers, defrosting devices and heaters so as to avoid visibility problems, which arise particularly in the cold seasons and under rainy conditions. However, it is not commercially feasible to install this equipment on the side windows of a vehicle, or on the rearview mirrors arranged outside of the vehicle. Similarly, it is difficult, if possible at all, to mount such antifogging equipment on windowpanes of buildings, lenses of eyeglasses and endoscopes, dental mouth mirrors, shields of masks and helmets, or cover glasses of measuring instruments.
As is well-known, a simple and convenient antifogging method conventionally used in the art is to apply onto a surface an antifogging composition containing either a hydrophilic compound such as polyethylene glycol or a hydrophobic or water-repellent compound such as silicone. However, the disadvantage of this method is that the antifogging coating thus formed is only temporary in nature and is readily removed when rubbed or washed with water so that its effectiveness is prematurely lost.
Japanese Utility Model Kokai Publication No. 3-129357 (Mitsubishi Rayon) discloses an antifogging method for a mirror wherein the surface of a substrate is provided with a polymer layer and the layer is subjected to irradiation by ultraviolet light, followed by treatment with an aqueous alkaline solution to thereby form acid radicals at a high density whereby the surface of the polymer layer is rendered hydrophilic. Again, however, it is believed that, according to this method, the hydrophilic property of the surface is degraded as time elapses because of adherent contaminants so that the antifogging function is lost over time.
Japanese Utility Model Kokai Publication No. 5-68006 (Stanley Electric) discloses an antifogging film made of a graftcopolymer of an acrylic monomer having hydrophilic groups and a monomer having hydrophobic groups. The graftcopolymer is described as having a contact angle with water of about 50.degree.. It is therefore believed that this antifogging film does not exhibit a sufficient antifogging capability.
Isao Kaetsu "Antifogging Coating Techniques for Glass", Modern Coating Techniques, pages 237-249, published by Sogo Gijutsu Center (1986), describes various antifogging techniques used in the prior art. The author Mr. Kaetsu nevertheless reports that the prior art antifogging techniques, which consist of rendering a surface hydrophilic, suffer from significant problems which must be overcome in reducing them to practice, and, further reports that the conventional antifogging coating techniques seemingly come up against a barrier.
Accordingly, an object of the invention is to provide an antifogging method which is capable of realizing a high degree of visibility in a transparent substrate such as a mirror, lens or glass.
Another object of the invention is to provide an antifogging method wherein the surface of a transparent substrate such as a mirror, lens or glass is maintained highly hydrophilic for an extended period of time.
Still another object of the invention is to provide an antifogging method wherein the surface of a transparent substrate such as a mirror, lens and glass is almost permanently maintained highly hydrophilic.
A further object of the invention is to provide an antifogging coating which has an improved durability and abrasion resistance.
Another object of the invention is to provide an antifogging coating which can be readily applied onto a surface requiring antifogging treatment.
Yet another object of the invention is to provide an antifogging transparent substrate such as a mirror, lens or glass, as well as a method of making such an antifogging transparent substrate, wherein the substrate surface is maintained highly hydrophilic for an extended period of time to thereby provide a high degree of antifogging property for an extended period.
In the fields of architecture and painting, it has been pointed out that growing environmental pollution tends to inadvertently accelerate fouling, contamination or soiling of exterior building materials, including outdoor buildings themselves and the coatings thereon.
In this regard, air-borne grime and dust particles are allowed under fair weather conditions to fall and deposit on roofs and outer walls of buildings. When it rains, the deposits are washed away by rainwater and are caused to flow along the outer walls of the buildings. Furthermore, air-borne grime is captured by rain and is carried onto surfaces (such as outer walls) of outdoor structures and buildings, where the grime may flow along or down the surface. For these reasons, contaminant substances are caused to adhere onto the surface along the paths of rainwater. As the surface is dried, a striped pattern of dirt, stain or smudge will appear on the surface.
The dirt or stain thus formed on the exterior building materials and exterior coatings consists of contaminant substances which include combustion products such as carbon black, city grime, and inorganic substances such as clay particles. The diversity of the fouling substances is considered to make the antifouling countermeasures complicated (Yoshinori KITSUTAKA, "Accelerated Test Method For Soiling on Finishing Materials of External Walls", Bulletin of Japan Architecture Society, vol. 404 (Oct. 1989), pages 15-24).
Hitherto, it has been commonly considered in the art that water-repellent paints such as those containing polytetra-fluoroethylene (PTFE) are desirable to prevent fouling or soiling of exterior building materials and the like. Recently, however, it is pointed out that, in order to cope with city grime containing a large amount of oleophilic components, it is rather desirable to render the surface of coatings as hydrophilic as possible ("Highpolymer", vol. 44, May 1995, page 307).
Accordingly, it has been proposed in the art to coat a building with a hydrophilic graftcopolymer (Newspaper "Daily Chemical Industry", Jan. 30, 1995). Reportedly, the coating film presents a hydrophilicity of 30-40.degree. in terms of the contact angle with water.
However, in view of the fact that inorganic dusts, which may typically be represented by clay minerals, have a contact angle with water ranging from 20.degree. to 50.degree. (so that they have affinity for graftcopolymer having a contact angle with water of 30-40.degree.), it is considered that such inorganic dusts are apt to adhere to the surface of the graftcopolymer coating and, hence, the coating is not able to prevent fouling or contamination by inorganic dusts.
Also available in the market are various hydrophilic paints which comprise acrylic resin, acryl-silicone resin, aqueous silicone, block copolymers of silicone resin and acrylic resin, acryl-styrene resin, ethylene oxides of sorbitan fatty acid, esters of sorbitan fatty acid, acetates of urethane, cross-linked urethane of polycarbonatediol and/or polyisocyanate, or cross-linked polymers of alkylester polyacrylate. However, since the contact angle with water of these hydrophilic paints is as large as 50-70.degree., they are not suitable to effectively prevent fouling by city grimes which contain large amount of oleophilic components.
Accordingly, a further object of the invention is to provide a method for rendering a surface of a substrate highly hydrophilic and antifouling.
Another object of the invention is to provide a method wherein the surface of buildings, window glasses, machinery or articles is rendered highly hydrophilic to thereby prevent fouling of or to permit self-cleaning of or to facilitate cleaning of the surface.
Yet another object of the invention is to provide a highly hydrophilic antifouling substrate, as well as a method of making thereof, which is adapted to prevent fouling of or to permit self-cleaning of or to facilitate cleaning of the surface.
In certain apparatus, formation of moisture condensate on a surface thereof often hampers operation of the apparatus when condensate has grown into droplets. In heat exchangers, for example, the heat exchanging efficiency would be lowered if condensate particles adhering to radiator fins have grown into large droplets.
Accordingly, another object of the invention is to provide a method for preventing adherent moisture condensate from growing into larger water droplets wherein a surface is made highly hydrophilic to thereby permit adherent moisture condensate to spread into a water film.